1. Field of the Invention
The present invention relates to an image processing method adopted to remove noise contained in an image.
2. Description of Related Art
Noise removal processing is executed in the related art by smoothing the target image in the final output space. However, since the noise fluctuation width usually changes depending upon the brightness level and the noise signal strength distribution is such that noise signals are mixed in with the brightness component in the final output color space, it can be extremely difficult to extract the noise component from image signals expressed with an aggregation of signals assuming various brightness levels by eliminating the influence of the various brightness levels. Some of the measures taken in the related art to address this issue, such as the adjustment of the noise removal filter coefficient or the filtering range of the noise removal filter in correspondence to the brightness level greatly complicate the processing (see patent reference 1). Other measures proposed in the related art include adjustment of the extent of noise removal in correspondence to the brightness level (see patent references 2, 3, 4 and 5).
There are methods having been proposed to address the issues discussed above more effectively, so as to achieve accurate noise removal while minimizing the inherent problems, by first projecting the target image into an image processing space optimal for noise removal and removing the noise from the image in the image processing space. For instance, patent reference 6 discloses a uniform noise space suitable for noise extraction and patent reference 7 proposes a uniform color uniform noise space achieved as an upgrade over the uniform noise space, which assures both accurate noise removal and effective prevention of the problems discussed above. It is to be noted that there are publications that, although not directly addressing the issues, disclose uniform color spaces optimized for human perception characteristics, each proposed to be used as an image processing space for noise removal (see patent reference 8 and non-patent reference 1).
Patent reference 1: Patent Publication No. 3809818
Patent reference 2: U.S. Pat. No. 5,051,842
Patent reference 3: U.S. Pat. No. 6,754,398
Patent reference 4: Patent Publication No. 3723043
Patent reference 5: Patent Publication No. 2650759
Patent reference 6: International Publication No. 2006/006373 pamphlet
Patent reference 7: International Publication No. 2006/064913 pamphlet
Patent reference 8: Japanese Laid Open Patent Publication No. H11-187420
Nonpatent reference 1: C. Tomasi et al., “Bilateral Filtering for Gray and Color Images” Proceedings of the 1998 IEEE International Conference on Computer Vision, Bombay, India